Conventionally, thick damascene copper wires (e.g., 3 microns or thicker) exhibit significantly greater surface roughness, as well as grain morphology (e.g., grain growth) than thinner damascene copper wires (e.g., 1 micron or thinner). One reason for this difference is that the thicker copper wires are not constrained by the wire height.
Surface roughness in thick damascene copper wiring can contribute to diminished cap adhesion (e.g., of a silicon nitride, SiN cap) and other problems during subsequent thermal processing. This diminished cap adhesion can cause undesirable conditions such as delamination of the later-formed nitride/oxide film over the copper wiring, which may lead to via opens.